1. Field of the Invention
The present invention relates to a primary side control (PSC) switching-mode power supply (SMPS), and particularly to a PSC SMPS that has reduced output voltage jitter.
2. Description of the Prior Art
Power supplies are a necessary electronic device in most electronic products, and are used for converting battery or grid power to power required by the electronic product and having specific characteristics. In most power supplies, switching-mode power supplies have superior electrical energy conversion efficiency and smaller product dimensions, making them popular in the power supply market.
Two different control schemes are used in current switching-mode power supplies: primary side control (PSC) and secondary side control (SSC). SSC directly couples a detection circuit to an output node of a secondary winding of a power supply, then through a photo coupler, transmits a detection result to a power supply controller located on the primary side to control energy of the power supply that is to be stored and converted on the primary winding. Compared to SSC, PSC indirectly detects voltage outputted by the secondary winding through directly detecting reflected voltage on an auxiliary winding, and indirectly completes detection of output voltage on an output node of the power supply. PSC completes detection and energy conversion control on the primary side. Compared to SSC, PSC is able to lower cost, as PSC does not require the photo coupler having both greater size and cost. PSC may also have higher conversion efficiency, because PSC does not require the detection circuit on the secondary side that constantly drains energy.
FIG. 1 is a diagram of a switching-mode power supply that uses PSC. Bridge rectifier 20 rectifies alternating current from grid node AC to establish direct current input power at input node IN. Voltage VIN of output power may have an M-shaped waveform, but may also be filtered into a fixed level that roughly does not vary over time. Transformer has three windings: primary winding PRM, secondary winding SEC, and auxiliary winding AUX. Power supply controller 26 periodically controls power switch 34 through gate node GATE. When power switch 34 is ON, primary winding PRM performs energy storage. When power switch 34 is OFF, secondary winding SEC and auxiliary winding AUX discharge to establish output voltage VOUT on output node OUT for supply to load 24, and control voltage VCC for supply to power supply controller 26.
Voltage divider resistors 28, 30 detect voltage VAUX of auxiliary winding AUX to provide feedback voltage VFB to feedback node FB of power supply controller 26. According to feedback voltage VFB, power supply controller 26 establishes compensation voltage VCOM on compensation capacitor 32, and controls power switch 34 according thereto.
FIG. 2 shows the power supply controller 26 of FIG. 1 and some external components. Power supply controller 26 comprises sampler 12, pulse generator 14, transconductor 15, and pulse width controller 16. During discharging of secondary winding SEC and auxiliary winding AUX, pulse generator 14 provides a short pulse to sampler 12, so that sampler 12 samples feedback voltage VFB to generate feedback voltage VIFB at intermediate node IFB. Through feedback node FB, voltage divider resistors 28 and 30, and auxiliary winding AUX, feedback voltage VIFB equivalently represents voltage level of secondary winding voltage VSEC of secondary winding SEC during discharging, and roughly represents output voltage VOUT. Transconductor 15 controls compensation voltage VCOM on compensation node COMP according to a comparison result of feedback voltage VIFB and target voltage VREF. Pulse width controller 16 controls power switch 34 according to compensation voltage VCOM. Overall, power supply controller 26 provides a feedback mechanism that roughly stabilizes feedback voltage VIFB to target voltage VREF, and is thus able to stabilize output voltage VOUT.